In telecommunication optical fibres have been used for several years, primarily owing to their large reliability, their insensitivity to electrical interference and their high capacity. Of course, there is a desire in the existing telecommunication networks to use the available optical fibres in their networks as efficiently as possible, in particular for communication over long distances, since such fibres obviously have high installation costs. By introducing wavelength division multiplexing WDM in existing communication systems using optical fibres a plurality of individual wavelength channels can be transmitted on the same optical fibre and thus the information transmitted over the fibre can be multiplied. Thus the need for installing more optical fibres can be postponed. Also, the telecommunication operators of course want to utilize their existing transmission equipment if possible also when changing to WDM systems or at least to utilize their existing equipment to the highest possible extent.
When using WDM in a link built of a single optical fibre pair between two nodes all information from one node to the other one will be transmitted at each instant over on one of the fibres of the pair. Because of the very large information amount transmitted over the fibre pair, a break-down of such a link will be extremely embarrassing. Thus, the ability of a network to restore communication or traffic on a failed link is very important. Protection must be built into links and networks using optical fibres carrying several WDM channels on optical fibres therebetween. Typical devices, in which failures can arise are of course the fibres themselves which can be cut off and the components in the transmission and receiving equipment.
In U.S. Pat. No. 5,299,293 a protection arrangement is disclosed which can be used in a WDM network. For the case of a faulty electrooptical transmitter, the input signal of the transmitter is coupled to the input of a spare transmitter through a n: 1 electrical switch. The spare transmitter includes a tunable laser adapted to transmit the signal on the same wavelength as used by the defective transmitter. In U.S. Pat. No. 5,457,556 equipment for protecting optical communication to failures of the WDM equipment are disclosed. The published German patent application 44 33 031 discloses redirection of the information flow in an optical line to another line when the first line gets defective. In U.S. Pat. No. 5,218,465 is described how traffic for some failure can be redirected to another redundant path. A cross-connect switch receives all the input signals and is controllable to switch each signal to the redundant path.
In the published International patent application WO 97/45977 “Channel protection in data-communication and telecommunication systems” an optical fiber network is disclosed using WDM, in which each node comprises at least one standby electrooptical transmitter and at least one standby optoelectrical receiver. A spare wavelength is used by the standby transmitter and receiver. The network is the bus type having traffic circulating through the node, in which the nodes tap off and/or add WDM channels as required.